SPD/SD DIAGNOSIS Berichtzusammenfassung

The use of prospective longitudinal surveys in improving the understanding of SAV infections of farmed salmon and freshwater rainbow trout

Initially, Partner 1 performed a retrospective study on Atlantic salmon on two sites in Ireland. The results of this indicated that virology, serology and histopathology could be used to follow the course of infection and to gain information on the duration of viraemia and the development and persistence of VN antibodies and the temporal relationship between these and histopathological changes and clinical signs. To build on these findings, three prospective studies were initiated by Partner 1 to study in detail the epidemiology of SAV infections in both trout and salmon. Trout in the first study did not become infected, but infections occurred on both other occasions.

A prospective longitudinal survey for sleeping disease (SD) was carried out over a 20-week period on a caged freshwater population of farmed rainbow trout (Oncorhynchus mkyiss) in the second study. Pancreas, heart and red and white skeletal muscle were examined histologically and presence and severity of lesions recorded. Sera were tested for viraemia with Salmonid Alphavirus (SAV) and for virus neutralizing (VN) antibodies.Viraemia was detected for 4 weeks, beginning at week 6 and with a peak prevalence of 57.9% at week 7. Clinical signs and mortalities appeared at week 8. Total mortality in the study cage from week 6 onward was 6.3%, but other cages on the site had mortality levels of up to 47.2%. VN antibodies were first detected at week 9, with seroprevalence increasing to 80% at the end of the study. Geometric mean titres peaked at 1/89.4 at week 17. Histological lesions were first detected at week 7 (pancreas only), before increasing in prevalence and severity to peak at weeks 9 and 10. The majority of lesions were resolved by week 15.

A prospective longitudinal study of salmonid alphavirus infection in farmed Atlantic salmon (Salmo salar L.) was initiated in post-transfer smolts on a UK farm in July 2004 and continued for 320 days in the third study. Sampling was concentrated on a single caged population (C4) with serum and tissue sampled collected and tested for viraemia, virus neutralizing (VN) antibodies, viral nucleic acid by real time RT-PCR and by histopathology. 380 sera collected between day 0 (d0) and d139 were consistently negative for both viraemia and VN antibodies. The first evidence of infection was detected at d146, when 2/20 fish were found to be viraemic and 1/20 to be antibody positive. At d153 only 1/20 fish were viraemic and one antibody positive. At the next sampling (d168) no viraemic or antibody positive fish were detected. Thereafter, single viraemic fish were detected on four occasions, including d320. The prevalence of antibody-positive fish remained low (0-5%) until d192 after which time it rose irregularly to a peak of 57.9% at d320. Real time RT-PCR was found to be more sensitive that screening for viraemia detecting a peak of 35% positive at d153 before declining. Histological lesions diagnostic for pancreas disease (PD) were observed at d146 and 153 only. Mild cardiac and to a lesser extent brain lesions were found frequently both before and after virus was detected. No clinical signs or mortalities attributable to PD occurred throughout the study. This is the first detailed report of sub-clinical infection.

The results from these studies highlights the usefulness of longitudinal surveys and the techniques for detection of virus and antibodies developed within the project as diagnostic and epidemiological tools.